This invention relates generally to integrated circuit memories, and more particularly to establishing a reference voltage in a memory.
Many memory types, such as EPROM (electrically programmable read only memory), flash, MRAM (magnetoresistive random access memory), and one-transistor DRAM (dynamic random access memory) are single-ended. That is, each memory cell is sensed using a single bit line. This is unlike an SRAM (static random access memory), where each memory cell is connected to a bit line pair. To allow differential sensing in memory types that traditionally use single-ended sensing, a reference voltage or current may be established with which to compare the state stored in a selected memory cell. The reference voltage or current is generally about half-way between a logic high voltage and a logic low voltage of the memory cell.
Several techniques have been used in the past to generate the reference voltage for memories using voltage sensing. One technique used to generate the reference voltage depends on the use of xe2x80x9cdummy cellsxe2x80x9d. A dummy cell is manufactured using the same process technology as the normal cells of a memory array in order to model the behavior of the normal cells as closely as possible. However, the dummy cell will be physically smaller to generate a reference voltage that is between a logic high voltage and a logic low voltage for the cell. The problem with this technique is that reducing geometries of the cells produces process problems in keeping the current ratio of the dummy cell relative to the normal cell constant.
Another technique for producing the reference voltage is to connect normal sized dummy cells in series or parallel combinations. One of the cells will be programmed to read a xe2x80x9czeroxe2x80x9d state and the other programmed to read a xe2x80x9conexe2x80x9d logic state to produce the required reference voltage. However, this technique may create errors due to the non-linearity of the cell resistance with voltage.
Yet another technique involves the use of current mirrors to establish the reference voltage. However, current mirrors sometimes do not produce the desired current with an acceptable degree of accuracy.
Therefore, there is a need for a circuit that can generate a more accurate reference voltage in a memory cell that uses single-ended sensing.